1. Technical Field
The present invention relates to a rotary angle detecting device suitable for detecting, for example, a rotary angle of a detection target such as a steering shaft of a vehicle.
2. Related Art
For example, in a posture control technology of a vehicle, a rotary angle detecting device is suitable for detecting a rotary angle of a steering shaft, rotating in accordance with an operator's steering operation, as an absolute angle. For the rotary angle detection, there is generally known a method in which a main driving gear rotates in accordance with a rotation of a steering shaft and an absolute angle is calculated on the basis of a phase difference of rotation occurring in plural driven gears meshing with the main driving gear. Particularly, upon detecting the rotary angle, the driven gears need to rotate at the low torque without a load. For this reason, a predetermined clearance needs to be ensured between the driven gears and the bearing part thereof. However, when such a clearance is provided, a problem arises in that the detection precision decreases due to a rattling movement occurring in the driven gears in an axial direction in a vehicle running state or an abnormal noise occurs due to a vibration.
For this reason, a technique is known in which a driven gear is elastically pressed in an axial direction so as to suppress a rattling movement or a vibration sound in a vehicle running state (for example, see Japanese Patent Application Laid-Open No. 2004-279265, pages 4 to 5, and FIGS. 1 and 4). The known technique has a configuration in which a driven gear having a magnet provided at the center thereof is accommodated in a case member, and a bearing concave portion is provided in the inner surface of the case member so as to rotatably support one end of the driven gear. In addition, in the case member, a circuit board is disposed in the other end of the driven gear so as to face a magnet, and a magnetic variation detecting element is mounted on the circuit board so as to detect a variation in magnetic field. Particularly, in the known technique, a spring member is installed between the case member and one end of the driven gear so as to press the driven gear against the circuit board by means of the elastic force thereof.
According to the known technique, even when a clearance is provided between the bearing concave portion or the circuit board and the driven gear, it is supposed that the driven gear can rotate at the low torque in the state where the rattling movement or the vibration of the driven gear is suppressed.
However, in the known technique, since the circuit board and the mechanism component such as the main driving gear and the driven gear are disposed in the same space inside the case member, a solder ball or residual flux may be separated from the circuit board or foreign materials such as broken pieces or fine powder may be separated from a broken surface of the circuit board due to a long-time use or a vibration in a vehicle running state. Then, the foreign materials are attached to the meshing part where the main driving gear meshes with the driven gear or the sliding part where the respective gears slide on the circuit board or the like. As a result, a problem arises in that the gears cannot smoothly rotate due to the large rotary torques of the gears or the rotary angle cannot be correctly detected due to the broken gear teeth.